1. Field of the Invention
The present invention relates to a cold-cathode electron source for emitting an electron beam, a microwave tube using it, and a production method thereof.
2. Related Background Art
Conventionally, a microwave tube such as a traveling-wave tube (TWT) or a klystron uses a focusing type hot-cathode electron source or a cold-cathode electron source having microscopic emitters of conical shape, and a cold cathode is disclosed, for example, in Non-patent Document 1 below or other documents. In general, this cold cathode (a cathode electrode and emitters (electron emitting electrodes)) is made up of such a material as a refractory metal material, e.g., tungsten or molybdenum, or a semiconductor material, e.g., silicon.
A commonly known method of constructing this microwave tube so as to support an operation at higher frequencies is to decrease capacitances between a gate electrode for adjusting the amount of electrons emitted from the emitters, and the emitters and between the gate electrode and the cathode electrode. In the cold-cathode electron source 50 disclosed in Non-patent Document 2 below, an insulating layer 52 is thickened to set the gate electrode 54 apart from the cathode electrode 56, thereby decreasing the capacitance between the gate electrode 54 and the cathode electrode 56 (cf. FIG. 9). This cold-cathode electron source 50 adopts the emitter shape in which only a part of the upper end of emitter 58 is tapered and in which the remaining major part is maintained in a thick circular cylinder shape, whereby the current density of an electric current flowing in the emitter 58 is lowered to prevent melting of the emitter 58.
Another example of the reduction of the capacitance between the gate electrode and the cathode electrode and the like is the cold-cathode electron source disclosed in Patent Document 1 below, and in this cold-cathode electron source 60 the insulating layer 64 is thickened stepwise with distance from the emitters 62, thereby decreasing the capacitance between the gate electrode 66 and the emitters 62 and the capacitance between the gate electrode 66 and the cathode electrode 68 (cf. FIG. 10).
[Patent Document 1] Japanese Patent Application Laid-Open No. 9-82248
[Patent Document 2] Japanese Patent Application Laid-Open No. 2001-202871
[Patent Document 3] Japanese Patent Application Laid-Open No. 8-255558
[Non-patent Document 1] Nicol E. McGruer, A Thin-Film Field-Emission Cathode, “Journal of Applied Physics,” 39 (1968), p. 3504-3505
[Non-patent Document 2] Nicol E. McGruer, Prospects for a 1-THz Vacuum Microelectronic Microstrip Amplifier, “IEEE Transactions on Electron Devices,” 38 (1991), p. 666-671